Description:
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Using a UTF-8 String as input for a com-function will generate a wrong string in COM interface. If the input string is containing n-chars which are encoded with 2 bytes, the length of resulting string is n byte too long. 

example: "I want to Dusseldorf and Koln" is correctly handled
"I want to Düsseldorf and Köln" will call COM function with a string:
"I want to Düsseldorf and Köln\0\4"  

reason:

ext/com_dotnet/com_variant.c

...
PHP_COM_DOTNET_API void php_com_variant_from_zval(VARIANT *v, zval *z, int codepage TSRMLS_DC)

...

case IS_STRING:
                        V_VT(v) = VT_BSTR;
                        olestring = php_com_string_to_olestring(Z_STRVAL_P(z), Z_STRLEN_P(z), codepage TSRMLS_CC);
                        
here is the problem:

V_BSTR(v) = SysAllocStringByteLen((char*)olestring, Z_STRLEN_P(z) * sizeof(OLECHAR));

When input string is UTF-8 encoded Z_STRLEN_P(z) has a count of 2 byte for each "special" char. So length of input string is count of all chars + count of "special" chars. After conversion to olestring Z_STRLEN_P(z) is the wrong length the olestring is shorter. So SysAllocStringByteLen is allocating too much memory and while the for the string length of olestring. In result those strings are always showing a '\0' for the first special char + n-1 random chars for the remaing special chars.